This invention relates to cleaning of heat exchange apparatus. More specifically, it relates to the removal of solids which accumulate on relatively cold surfaces of a heat exchanger, or cooler, in which a fluid stream is cooled by a gas stream.
In the cooling of fluids consisting of several components, one (or more) of which has a relatively high melting point, it is common that the high melting point component accumulates on relatively cold surfaces of the cooler. Deposits may form because the local film temperature is low or solid or semi-solid particles in the fluid stream may stick to surfaces upon which they impinge. The problem is magnified at surfaces where the fluid velocity is low. These accumulations or deposits reduce the amount of heat transferred from the fluid to the cooling media when they form directly on the heat transfer surface and increase the pressure drop across the cooler, regardless of whether they form on active transfer surface or elsewhere in the flow paths of the cooler. In order to remove deposits, either mechanically or by adding steam to the apparatus, the cooler must be taken out of service. Since it is seldom that spare heat exchangers are installed, the necessity for cleaning often requires shutting down a complete production unit.
Certain operations in a petroleum refinery are particularly susceptible to solids depositing in coolers. Among these operations are hydrocracking of distillates and heavy gas oils. The deposits may comprise paraffin waxes and condensed ring aromatics. Another example of a problem area in a refinery is deasphalting, where asphalt droplets entrained in a process stream solidify and accumulate in the cooler when the stream is cooled. Refinery units normally operate on a continuous basis, shutting down only once a year for maintenance and repair. To avoid a shut-down, adverse operating conditions may be accepted as the lesser of the two evils. For example, a higher temperature stream from a cooler will have undesirable effects on a vapor-liquid separator and a compressor, which are commonly located downstream in a refinery. It is usually desirable to run both of these pieces of equipment at as low a temperature as possible, in order to increase yield of product and decrease maintenance expense.